[gn] Automatically derive build configs in test runner.

tools/testrunner/utils/dump_build_config.py

Index: tools/testrunner/utils/dump_build_config.py
diff --git a/tools/testrunner/utils/dump_build_config.py b/tools/testrunner/utils/dump_build_config.py
new file mode 100644
index 0000000000000000000000000000000000000000..01acadc123955d71f3215d648e4e07b37cc50008
--- /dev/null
+++ b/tools/testrunner/utils/dump_build_config.py
@@ -0,0 +1,28 @@
+# Copyright 2016 The V8 project authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+"""Writes a dictionary to a json file with the passed key-value pairs.
+
+Expected to be called like:
+dump_build_config.py path/to/file.json [key1=value1 ...]
+
+The values are expected to be valid json. E.g. true is a boolean and "true" is
+the string "true".
+"""
+
+import json
+import os
+import sys
+
+assert len(sys.argv) > 1
+assert os.path.exists(sys.argv[1])
+assert os.path.isdir(sys.argv[1])
+
+def as_json(kv):
+  assert '=' in kv
+  k, v = kv.split('=', 1)
+  return k, json.loads(v)
+
+with open(os.path.join(sys.argv[1], 'v8_build_config.json'), 'w') as f:

[vogelheim, 2016/06/30 11:08:02]

os.path.join(...): This is not what the header comment says. Why not make
sys.argv[1] the actual full file name, rather than a path w/ an implied name?

[Michael Achenbach, 2016/06/30 11:47:50]

Done. But as a consequence I needed to remove the assertions. I can't call
dirname on sys.argv[1] because the script is executed in the outdir and there
are no slashes passed by gn. Just the name of the json file. Before, gn passed
the weird construction ../Release if Release was the out dir.

[vogelheim, 2016/06/30 12:09:39]

I guess you could use os.path.absname to first construct the absolute path, but
I don't think there's much value in that...

+  json.dump(dict(as_json(kv) for kv in sys.argv[2:]), f)

[vogelheim, 2016/06/30 11:08:02]

please add a try-except for IOError, and return -1 (or however that works in
python.)

(Because: Since this is meant to run as part of the build, we should make sure
it will return non-zero value in case of errors.)

[Michael Achenbach, 2016/06/30 11:47:49]

Not sure what you mean. When an exception is thrown to top-level, the return
code will be -1. I just tried to add some wrong json in the build file and then
this step fails nicely.

Or do you want to avoid seeing the stack trace of the exception?

[vogelheim, 2016/06/30 12:09:39]

Ah, that is what I meant. I had assumed an uncaught exception to do something
different. :)